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The first thing I want to show you is the flexible material, because this stuff is very easy to use, so it’s more costly than the straight nonflexible flashing. The first thing I do when I do a sill pan is I precut the material so it wraps up the sides of the jamb somewhere between 4 and 6 inches high. Then I want to remove just part of the release sheet.
Work from the inside, out
Many of the manufacturers have pre-embossed strips along the back, so you don’t have to remove all of the release sheet, just part of it. So I’m going to remove the main part of the back and leave the last strip on the front. I find it’s easiest to work with the flashing tape from the inside of the rough opening rather than the outside, but if you find differently, by all means; it can be done either way.
Even with flexible flashings, corners are critical
It’s important to get a nice, tight press on the inside corner where the sill meets the side of the rough opening. Otherwise, if you bridge that corner there, you can end up breaking the flashing tape itself when the windows get set in place. Once the sill is set in place, now we can remove the outer piece of release sheet.